The typical way of producing an ozone containing gas is by corona discharge. Typically, oxygen-containing gas, such as air or substantially pure oxygen (i.e. gas having an oxygen concentration of at least about 97%), is passed through the discharge gap of a corona discharge cell, such as shown in FIG. 2 of "Design Considerations for Ozone Bleaching", Byrd and Knoernschild, TAPPI Journal, May, 1992, pages 101-106. In a corona discharge cell ozone is formed by the following reactions: EQU O.sub.2 +2e.fwdarw.20.sup.- EQU 20.sup.- +20.sub.2 .fwdarw.20.sub.3 +2e.sup.-
Heat is generated during these reactions, and to prevent thermal decomposition of the ozone, this heat must be removed from the electrodes. Water, oil, or a like cooling liquid is used to cool the generator elements. Power is supplied to the high voltage electrode by means of an AC transformer.
In order to maximize the efficiency of ozone generation and cooling, tube type corona discharge cells have been developed, such as shown in U.S. Pat. No. 4,988,484, the disclosure of which is hereby incorporated by reference herein. Conventional tube-type ozone generators have an inner electrode, an outer electrode, a dielectric between the electrodes, either on the inner electrode or the outer electrode, and an annular passageway defined between the electrodes and dielectric. The annular passageway is typically linear and elongated in a dimension of elongation. An inner passageway is also provided interior of the inner electrode and substantially concentric therewith, and a cooling fluid (such as nitrogen gas, water, oil, or other cooling fluid) passes over the external electrode and through the interior passageway. The tubes are typically enclosed within a housing, and are disposed in banks, such as seen in FIG. 3 of U.S. Pat. No. 4,988,484.
In conventional tube-type ozone generators, oxygen-containing gas is introduced into the annular passageway so that it flows in the dimension of elongation of the annular passageway (i.e. makes substantially a 0.degree. angle with respect to the dimension of elongation). It is heretofore not been recognized that the manner in which the oxygen-containing gas is introduced into the tube-type generator could have any effect on the generator's power efficiency, or on the ability of a given generator to produce a particular concentration of ozone in the product gas. However, surprisingly, according to the present invention it has been found that the manner in which the gas is introduced into the annular passageway, and flows in the passageway, can have dramatic effect on power efficiency and/or the concentration of ozone in the product gas.
There are some uses of ozone in which the efficiency of ozone generation and the amount of ozone in the product gas are important to the practical success of the process utilizing the ozone. For example in the bleaching of cellulose pulp with ozone, especially in the bleaching of medium consistency pulp such as shown in European patent publication 0397308, it is necessary to maximize both the efficiency of the ozone generation, and the amount of ozone that can be provided in the product gas stream. Typically, ozone concentrations of at least 8% in the product gas are desirable, while ozone concentrations of 10% or more are preferred.
Another use of ozone is in the purification of potable water and other water supplies.
According to the present invention it has been found that by simply passing the oxygen-containing gas through the annular passageway of a tube-type ozone generator so that the gas "swirls" in a cyclonic flow pattern as it passes through the annular passageway (that is, rotates around an axis defined by the dimension of elongation as it substantially linearly moves in the dimension of elongation), the power efficiency of the tube-type ozone generator can be dramatically improved, at least about 10%, and typically about 50% for the production of product gas having an ozone concentration of about 8%. Also, the simple swirling movement of the oxygen gas in the tube-type generator allows the same generator (with the same flow, power, and other conditions) to produce a product gas having a higher concentration than is possible under identical operating conditions if the swirling action (cyclonic flow pattern) is not utilized, ozone concentrations of 10 to 13%, or possibly even more, being achievable according to the invention.
According to one aspect of the present invention a method of generating ozone from an oxygen-containing gas utilizing a tube-type generator having an inner electrode, outer electrode, and dielectric associated with one or both of the electrodes, and defining an annular passageway elongated in a dimension of elongation, is provided. The method comprises the steps: (a) Passing oxygen-containing gas through the annular passageway so that the gas makes an angle of greater than 0.degree. and less than 90.degree. with respect to the dimension of elongation so that the oxygen-containing gas swirls as it travels in the dimension of elongation; and (b) applying electrical power to the electrodes to create a corona discharge to generate ozone from some of the oxygen in the oxygen-containing gas, a product gas having at least one percent ozone being discharged from the generator. Step (a) is typically practiced by passing essentially pure oxygen through the annular passageway, although ambient air can also be used as the oxygen-containing gas if low ozone concentrations are acceptable.
Step (a) is typically practiced so that the amount of power applied in step (b) to produce a given quantity of ozone in the product gas is at least about 10% less than if step (a) were practiced by introducing the oxygen-containing gas so that it flowed in the dimension of elongation, without swirling, with all other conditions the same. In fact, step (a) may be practiced so that when the product gas has an ozone concentration of about 8% the amount of power utilized in the practice of step (b) is roughly about 50% less than if step (a) were practiced by introducing the oxygen-containing gas so that it flowed in the dimension of elongation, without swirling, with all other conditions the same.
Optimum results are typically achieved if the oxygen-containing gas makes an angle of about 45.degree.-90.degree. with respect to the dimension of elongation. Step (b) is typically practiced by applying AC power having a frequency of at least about 50 Hz, for example about 50-2500 Hz (e.g. about 450 Hz). Steps (a) and (b) may be practiced so that the product gas has an ozone concentration of at least about 10%. The orientation of elongation of the annular passageway is not critical and may be horizontal, vertical, or anything in between.
Typically, the generator has an interior passageway substantially concentric with the inner electrode and there is the further step, commensurate with the prior art, of passing a cooling fluid, such as nitrogen gas, water, oil, etc., through the inner passageway, either co-current or countercurrent to the general direction of the oxygen gas flow through the annular passageway.
The invention also contemplates a method of generating ozone containing gas utilizing a tube-type ozone generator, comprising the following steps: (a) Introducing essentially pure oxygen gas into the passageway so that it flows from the first end of the passageway to the second end. (b) Applying AC power to the generator electrodes so as to produce a corona discharge causing some of the oxygen flowing in the passageway to be transformed to ozone, so that a product gas is produced having ozone therein. And, wherein steps (a) and (b) are practiced so that the product gas has an ozone concentration of at least about 8% at a power cost of less than about ten kw/kg/hr. The method is particularly suitable for generating ozone for use in ozone bleaching, including bleaching of medium consistency (e.g. about 8-15%) paper pulp, in which an ozone concentration of at least about 8% is desirable, and for which the power efficiency is important in order to achieve commercial practicality or optimization. In fact, step (a) may be practiced so that the power cost to produce product gas containing at least about 8% ozone is roughly about 50% of the power cost to produce product gas with that concentration of ozone using the same generator under the same conditions only with the oxygen gas flowing substantially in the direction of elongation without swirling.
According to yet another aspect of the present invention an ozone generator is provided comprising the following elements: An inner electrode. An outer electrode. A dielectric associated with one or both of the electrodes. The electrodes and dielectric defining an annular passageway having a first end and a second end spaced from each other in a direction of elongation. Means for introducing oxygen-containing gas into the first end of the annular passageway so that it makes an angle of greater than 0.degree. and less 90.degree. with respect to the direction of elongation, so that the introduced oxygen-containing gas swirls in the annular passageway as it passes from the first end to the second end thereof. And, means for applying AC power to the electrodes.
The means for introducing the oxygen-containing gas may comprise a nozzle, a plurality of nozzles, a header, slots, channels, orifices, or the like, disposed at an angle with respect to the direction of elongation corresponding generally to the angle that the gas assumes as it swirls in an annular passageway. Typically, the means for introducing the oxygen-containing gas comprises means for introducing the gas at an angle of about 45.degree. to 90.degree. with respect to the angle of elongation. As is conventional per se in the prior art, an interior passageway substantially concentric with an interior of the inner electrode is provided as well as means for circulating a cooling fluid through the interior passageway and externally of the outer electrode. The dielectric may be on the inner electrode, or on the outer electrode or on both electrodes, between the inner and outer electrodes. The electrodes are typically made of stainless steel and the dielectric is typically made of glass or ceramic, although other materials can be used for both. The means for applying AC power to the generator comprises means for applying power at a frequency of about 50-2500 Hz.
It is the primary object of the invention to produce ozone from oxygen-containing gas in an efficient manner, and/or to produce ozone containing gas having a higher concentration of ozone than is possible utilizing the same conditions and conventional equipment. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.